A bold new study claims mastodon fossils found in San Diego in 1992 show humans existed in North America 115,000 years sooner than previously thought. Here's why. (Gillian Brockell/The Washington Post)

Some 130,000 years ago, scientists say, a mysterious group of ancient people visited the coastline of what is now Southern California. More than 100,000 years before they were supposed to have arrived in the Americas, these unknown people used five heavy stones to break the bones of a mastodon. They cracked open femurs to suck out the marrow and, using the rocks as hammers, scored deep notches in the bone. When finished, they abandoned the materials in the soft, fine soil; one tusk planted upright in the ground like a single flag in the archaeological record. Then the people vanished.

This is the bold claim put forward by paleontologist Thomas Deméré and his colleagues in a paper published Wednesday in the journal Nature. The researchers say that the scratched-up mastodon fossils and large, chipped stones uncovered during excavation for a San Diego highway more than 20 years ago are evidence of an unknown hominin species, perhaps Homo erectus, Neanderthals, maybe even Homo sapiens.

If Deméré's analysis is accurate, it would set back the arrival date for hominins in the Americas and suggest that modern humans might not have been the first species to arrive. But the paper has raised skepticism among many researchers who study American prehistory. Several said this is a classic case of an extraordinary claim requiring extraordinary evidence — which they argue the Nature paper doesn’t provide.

“You can’t push human activity in the New World back 100,000 years based on evidence as inherently ambiguous as broken bones and nondescript stones,” said David Meltzer, an archaeologist at Southern Methodist University. “They need to do a better job showing nature could not be responsible for those bones and stones.”

For decades, discussion of early settlement of the Americas has focused on the tail end of the Ice Age. Most archaeologists agree that humans crossed a land bridge from Asia into Alaska sometime after 25,000 years ago, then either walked between ice sheets or took boats down the Pacific coastline to reach the wide open plains of Pleistocene America roughly 15,000 years before present. Though scientists debated the exact timing of this journey, their estimates differed by hundreds or a few thousand years, not tens of thousands.

When Cerutti spotted a broken tusk stuck in the soil overturned by an excavator, he called for a halt in activity and summoned Deméré to the site.

“You’ll want to see this,” Deméré recalled Cerutti saying.

The scientists set up a geographic grid system and began carefully excavating several more stones and bones, plotting each new object on their grid to preserve its location. It took several months to uncover every artifact.

“As the site unfolded over that five month period it became more and more exciting and more puzzling at the same time,” Deméré recalled.

The biggest find was a partial skeleton from a single American mastodon. Peculiarly, the largest bones were scarred and broken, but more fragile ribs and vertebrae were still intact. Some of the bones seemed to have been arranged deliberately alongside one another. Many bore the spiral fractures that are a signature of ancient people hammering on fresh bone — either to extract marrow for food or break the bone into tools.

The bones were clustered in groups around a few large, heavy stones known as “cobbles.” The size and makeup of these rocks didn’t match the fine-grained surrounding soil. They bore marks you'd expect to see on a hammer and anvil. Scattered around the site were flakes that seem to have been chipped off the cobbles, as though someone had struck the rocks against another solid object. When held up to their source stones, the flakes fit back into them like pieces of a puzzle.

“It was unusual to say the least … and suggested this was a not a typical paleontological site and we should consider the possibility that we had association of extinct megafauna with humans, or at least early human activity,” Deméré said of the findings.

But it was difficult to figure out how old the site was. Any soft tissue in the fossilized bones had long decayed, so scientists couldn’t use radiocarbon dating to determine their age. They attempted to date fossils using the uranium-thorium method, which measures radioactive decay of uranium. But the technique was not very reliable at the time, so the Cerutti mastodon remained an enigma.

More than a decade later, a mutual friend put Deméré in touch with archaeologist Steve Holen. Holen believes that human history in the Americas dates back much farther than the end of the Ice Age, something he acknowledges is a “minority position” in his field. For several years, he has been examining museum collections and new fossil sites in search of ancient bones that look like they were touched by people.

The breaks on the mastodon fossils looked as though they were human-caused, he said. But to make sure, Holen tried to recreate them using a stone hammer the same size as the one found at the Cerutti site and the skeleton of an elephant that had been recently buried.

“The bone was extremely fresh and smelled very bad,” Holen said of that experiment. “I almost wished I wasn't doing this.” It took all of Holen's effort — and the help of a younger, stronger colleague — to break the bones. When they succeeded, they recognized the same breakage patterns as the ones found on the fossils. There's no evidence that anyone hunted or butchered the mastodon for meat, but it definitely seemed to him like some human or human cousin had cracked the bones.

“Once you do the experiment then you really can understand this much better,” Holen said.

The surface of mastodon bone showing half impact notch on a segment of femur. (Tom Deméré, San Diego Natural History Museum)

Next the team reached out to geochronologist James Paces, who retried the now much-improved uranium-thorium dating technique on the bones. He concluded that they are 130,000 years old, give or take 9,400. This date corresponds with the accepted age of the layer of rock in which the bones and cobbles were found.

But it far exceeds any established date for settlement of the Americas. The oldest biological remains from any humans on the continent is a coprolite (fossilized poop) from 14,300 years ago. Studies based on genetic analysis of modern Native Americans suggest that humans didn't make it over the land bridge that once linked northeast Asia to Alaska until 25,000 years ago.

If the stones and bones really are evidence of people, then who were they? How did they get to this part of the world so long ago? And why haven't we found other evidence of their presence? Did they die out not long after they arrived?

A boulder discovered at the Cerutti Mastodon site thought to have been used by early humans as a hammerstone. (Tom Deméré, San Diego Natural History Museum)

Because there are no hominin remains at the site, and rock hammer technology was used by many hominin species, the scientists caution that discussion of the identity of these people is purely speculative. In a supplement to their Nature paper, they say the Cerutti people may have been Neanderthals, Denisovans (a species known only from a few fragments found in a cave in northern Siberia), or members of the species Homo erectus. It seems unlikely that they were Homo sapiens — anatomically modern humans didn't migrate out of Africa until after 100,000 years ago, according to most estimates.

As for how they got here, Deméré said they may have been able to cross the land bridge before the last ice age, when the planet warmed and sea levels rose. Other species migrated to the Americas in this period, Deméré said, and the hominins may have followed them over.

Otherwise, the first Americans could have used boats to cross the Bering Strait, and then scoot down the Pacific coast — archaeological finds on the Mediterranean island of Crete suggest that hominins were able to cross the sea via boat more than 100,000 years ago.

To some who study American prehistory, this interpretation of the Cerutti site beggars belief. Meltzer called the claim “grandiose.” Donald Grayson, a paleoanthropologist at the University of Washington, noted that history is rife with examples of scientists misinterpreting strange markings on stone as evidence of human activity. He pointed to the Calico Hills site in the Mojave Desert, which the archaeologist Louis Leakey believed contained 200,000-year-old stone tools. Subsequent studies have largely discredited Leakey's claim — the apparent tools were most likely “geofacts,” natural stone formations that only look like they were crafted by humans.

Some of the fossil bones and rocks found at the Cerutti mastodon site. (San Diego Natural History Museum)

“It is one thing to show that broken bones and modified rocks could have been produced by people, which Holen and his colleagues have done,” Grayson said. “It is quite another to show that people, and people alone, could have produced those modifications. This, Holen [has] most certainly not done, making this a very easy claim to dismiss.”

Mike Waters, the director of the Center for the Study of the First Americans at Texas A&M, also criticized the claim. To convince him that people were in the Americas so much earlier before the first physical evidence of their remains, he would expect to see “unequivocal stone artifacts,” he said. He doesn't think the cobbles found at the Cerutti mastodon site meet that standard.

Rick Potts, the director of the Human Origins Program at the National Museum of Natural History, was more measured in his appraisal. Though he thought the team's analysis of the bones and stones was thorough, he pointed out a few oddities about the site. For one, it's unusual that people would use hammer stones to process bones but not any sharp-edged tools, even though that technology had been around for more than a million years. For another, as he pointed out, the mastodon's molars were also crushed, and there's no reason he can think of that humans would crack the huge teeth. If those teeth were broken by natural forces, then perhaps the rest of the bones were too.

Briana Pobiner, a paleoanthropologist at NMNH who specializes in studying tooth and tool marks on ancient bones, agreed.

“It’s funny because when I first started reading the paper I didn’t see the extra zero and I thought, 'oh, 13,000 years, this sounds good,'" Pobiner said. “And then I saw the extra zero and I thought, 'Holy cow!'”

Pobiner acknowledged that the Cerutti site contains less archaeological evidence than scientists would like before making a claim of this magnitude. But as someone who has spent her whole career looking at scratch marks and breakage patterns on bones, the evidence looks to her like it could be human modification.

Deméré said that he and his colleagues considered possible alternate explanations, but none seemed to fit. Trampling by another large animal would not produce those breakage patterns, they concluded. And environmental forces, like a powerful flood, would have broken the smaller, more fragile bones as well as the big one. Holen added that the rock layer in which the artifacts were found is largely intact — it does not seem to have been subject to disturbances like earthquakes or upheavals that would make the site more difficult to interpret.

Erella Hovers, an archaeologist at Hebrew University in Jerusalem who reviewed the paper and wrote an analysis of it for Nature, said she thought the researchers did a thorough job of ruling out natural causes of the particular breakage patterns. She added that the evidence looks much like archaeological sites she has studied in Africa and the Middle East; if the same site was found in that part of the world, she said, people would have fewer questions about it.

The Cerutti site researchers expect to face scrutiny from his colleagues about the paper. That is partly why they have made 3-D images of the mastodon fossils available online.

“I think the models are important in terms of supporting the paper because they allow anyone to look at this evidence in much the same way the co-authors did,” co-author Adam Rountrey, collection manager at the University of Michigan Museum of Paleontology, said in a statement. “It’s fine to be skeptical, but look at the evidence and judge for yourself. That’s what we’re trying to encourage by making these models available.”

The scientists also hope that their paper will prompt their colleagues to take a closer look at this period in American history. Perhaps they will find more evidence of hominin presence, bolstering the Cerutti researchers' claim. Or perhaps the mastodon site is a fluke — or a mistake — and they will find nothing at all.

“The thing to remember is it's a beginning to a new line of inquiry. It doesn't solve anything,” said Hovers. “It asks new questions.”

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